The present invention relates generally to an image-formation optical system and an imaging system that incorporates the same. More particularly, the invention is concerned with imaging systems such as digital still cameras, digital video cameras, cellular phones, personal computers, small-format cameras mounted on motor cars, surveillance cameras or the like, all harnessing solid-state image pickup devices, e.g., CCDs or CMOSs,
In recent years, electronic cameras using solid-stage image pickup devices such as CCDs or CMOSs to take subject images have come into wide use in place of silver-halide film cameras. For imaging systems mounted on portable computers, cellular phones, etc. among those electronic cameras, size and weight reductions are especially demanded.
Some conventional image-formation optical systems used with such imaging systems are made up of one or two lenses. With those optical systems, however, any high performance is not expectable because of their inability to correct field curvature, as already known from discussions about aberrations. To achieve high performance, therefore, it is required to use three or more lenses.
However, even three-lens constructions have their own limitation in view of performance. For instance, when three-lens construction is applied to a CCD camera of as many as about 2,000,000 pixels, only marginal performance improvement is obtained even with recourse to some aspheric surfaces.
Thus, four-lens constructions comprising three lenses plus one lens are now envisaged. For a typical four-lens construction type, an inverted Ernostar type comprising +−++ lenses is known.
Prior inverted Ernostar types are typically known from the following patent publications 1, 2, 3, 4, 5, 6 and 7.
Patent Publication 1
JP-A 55-96915
Patent Publication 2
JP-A 63-199312
Patent Publication 3
JP-A 1-128025
Patent Publication 4
JP-A 2-137812
Patent Publication 5
JP-A 5-40220
Patent Publication 6
JP-A 2000-28914
Patent Publication 7
JP-A 2001-281539
With the conventional inverted Ernostar types, however, neither is any tradeoff between wide-angle arrangement and size reductions obtained, nor is any compromise achieved between size reductions and high performance.